1. Field of the Invention
This invention relates to improvements in a spark plug to be used in an internal combustion engine, and a method of producing the spark plug.
2. Description of the Prior Art
Hitherto, a variety of spark plugs for automotive internal combustion engines and the like have been proposed and put into practical use. Of these spark plugs, there is one in which a tip formed of an alloy whose main component is a noble metal is welded to an electrode so as to form an ignition section. Additionally, various attempts have been made to prevent the tip from falling off from the electrode.
In this regard, Japanese Patent Provisional Publication No. 62-268079 discloses the following technique: A ground electrode is formed with an insertion hole having such a shape that the tip cannot fall off toward a spark gap. The tip is inserted into the insertion hole in such a manner to form a depression defined by the tip and the ground electrode. The depression is filled with a lid member formed of the same alloy as that of the parent material of the ground electrode, and then resistance welding is made on the lid member and the ground electrode, thereby fixing the tip to the ground electrode.
Additionally, Japanese Patent Provisional Publication No. 2000-40577 discloses the following technique: Resistance welding is made from the side of a spark gap to fix the tip to a ground electrode, and then laser welding is made from the opposite side of the spark gap to fix the tip and the ground electrode.
However, in the former conventional technique, the tip cannot fall off toward the spark gap; however, the tip is merely pressed against the ground electrode by the lid member and therefore cannot be seemed to be firmly fixed to the ground electrode. In order to securely fix the tip to the ground electrode, a high dimensional precision is required for the lid member, the insertion hole and the tip, thereby raising machining cost. This is undesirable. Additionally, the tip and the ground electrode are not in completely tight contact with each other, and therefore thermal conduction between them is not so good. Particularly, this technique is accomplished by using the tip whose main component is Ir, heat of the tip cannot be transmitted to the side of the ground electrode, so that the temperature of the tip rises thereby particularly increasing the consumption of the tip owing to spark discharge.
In the latter conventional technique, the tip is fixed to the ground electrode only with molten (and solidified) sections formed by the resistance welding and the laser welding. In case that the tip is formed of an alloy whose main component is a noble metal (particularly, Ir), such a tip is largely different in melting point from a Ni-based alloy which is, in general, used as the patent material of the ground electrode, and therefore it is difficult to securely weld the tip and the ground electrode under the resistance welding. As a result, welding of the tip and the ground electrode substantially relies only on the laser welding, thereby rendering it difficult to completely prevent the tip from peeling off and falling off from the ground electrode.